desktoppackage: set nullish value to false

In Desktop.qml, a boolean property is set using values from the sidePanelStack, which may be null when nothing is loaded there, such as when plasmashell is started. The expression tests for this with optional chaining, but the js && operator returns the undefined value instead of converting it to a bool, and the assignment to the bool property triggers an error. This does not seem to have negative consequences, but produces unnecessary log spam.

This change explicitly sets nullish values to false, so property binding no longer causes an error message to be printed.